274 OX THE STRUCTURE OF THE SKULL. 



become greatly elongated and very rugged, the tympanic also 

 acquiring a very large size (Fig. 109). 



The vomer is a very long and large bone, deeply grooved 

 above for the ethmoidal cartilage, which extends downwards 

 and forwards between the premaxillae and the maxillae to near 

 the anterior end of the snout. Its expanded upper and pos- 

 terior end unites with the basi-sphenoid in the middle line, and 

 with the pterygoid laterally. In front of the basi-sphenoid it 

 embraces, not a distinct presphenoid (as in Pterobalsena, accord- 

 ing to Eschricht), but the inferior surfaces of the orbito- 

 sphenoids, which are very thick ; and, being applied together 

 by their flat median faces, apparently replace the proper pre- 

 sphenoid. 



Both these bones and the alisphenoids are small, and almost 

 confined to the base of the skull. 



The supra-occipital and inter-parietal are united together, 

 and completely overlap and hide the parietals in the roof of the 

 skull. The separate frontals only enter into the anterior wall 

 of the skull, and between them and the orbito-sphenoids an oval 

 aperture is left, doubtless diminished in the recent state by the 

 ethmoidal cartilage. Laterally, the frontals are prolonged 

 outwards and backwards into two great supra-orbital processes, 

 which nearly meet the zygomatic processes of the squamosal. 

 The short jugal bones, absent in the specimen figured, extend in 

 the Balsenoidea from the zygomatic process to the anterior and 

 external angles of the supra-orbital prolongations, and are dis- 

 tinct from the lachrvmals. 



The pterygoids are completely separated by the palatines 

 (Fig. 107). In front of the latter the maxillae almost wholly ex- 

 clude the premaxillaries from the palate, while they send great 

 processes obliquely outwards and backwards, in front of the 

 supra-orbital prolongations of the frontal. The long premaxillae, 

 on the other hand, pass upwards and backwards on each side of 

 the elongated and symmetrical nasals to meet the frontals, and 

 exclude the maxillae altogether from the anterior nares. 



The rami of the low r er jaw are very narrow, and so much 

 arched outwards as to be able to enclose the baleen plates 

 attached to the upper jaw when the mouth is shut. 



